The invention relates to a photoelectrochemical cell with a work electrode and a counter electrode which is arranged opposite the work electrode and whose electrochemically active surfaces are facing each other, with an electrolyte being arranged between the surfaces that contains a redox system, with the surface of the counter electrode being catalytically active. The invention furthermore relates to a method for producing a counter electrode for a photoelectrochemical cell.
A photoelectrochemical cell similar to the one described above is disclosed, for example, in EP 0 333 641 B1. The work electrode therein comprises a polycrystalline metal oxide semiconductor with a chromophore layer as the electrically active surface. Titanium oxide is the preferred choice for the metal oxide semiconductors and is applied to a glass panel that is coated with a conductive layer. For such photoelectrochemical cells, conductive coated glass, to whose surface a catalytically active material has been applied, is also used for the counter electrode. WO 97/12382 suggests the usage of platinum.
Another setup of a photoelectrochemical cell in which a carbon layer is used for the counter electrode is described in DE 195 40 712 A1. This design must generally have a large thickness to allow the flow of sufficient currents. Since the catalytic activity of carbon is low, it must have a large surface or be catalytically activated.
The counter electrode is an important component of the photoelectrochemical cell. The main requirement for the catalytically active material is that the transmission of electrons in the electrolytes must occur with the lowest amount of kinetic inhibition possible, i.e. that the exchange current density Io must be as high as possible for this process.
The current in a photoelectrochemical cell of the kind described above is generated when a dye that is adsorbed on the porous semiconductor injects electrons into the semiconductor when it is stimulated by light energy. This way, the electrons get to the external circuit through the conductive substrate coating and can perform their job there. The oxidized dye is reduced from the electrolyte through electrons, with the electrolyte filling the entire cell all the way to the counter electrode. The electrolyte generally contains a redox system, e.g. iodine/iodide, bromine/bromide or other systems, as they are described in EP 0 333 641 B1, for example.
In order to be able to close the circuit in the photoelectrochemical cell, the electrons arriving from the external circuit must be transmitted back into the electrolyte on the counter electrode. The catalytically active material of the counter electrode facilitates this process.
However, platinum, which is mentioned in WO 97/12382 as the catalytically active substance, has the disadvantage that it represents a relatively expensive component of the cell. WO 97/12382 emphasizes the combination of platinum with iodine/iodide electrolytes. Findings have now shown that the platinum dissolves through a reaction with iodine, a reaction which is very slow but which will shorten the life of the cell. Therefore, searches are in progress for alternatives to this catalyst, with the above-described carbon layers explained in DE 195 40 712 A1 not being regarded as the optimal choice.